Kinetic Particle Model of Matter — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

Section 1 of 22Properties of the three states

State	Shape	Volume	Flow	Compressible
Solid	Definite	Definite	No	No
Liquid	Takes the shape of its container	Definite	Yes	No
Gas	Takes the shape of its container	Fills the container	Yes	Highly

Section 2 of 22Changes of state

Changes of state

Melting

• Melting a solid turns to a liquid.

Freezing (solidification)

• Freezing (solidification) a liquid turns to a solid.

Boiling (vaporising)

• Boiling (vaporising) a liquid turns to a gas.

Condensing

• Condensing a gas turns to a liquid.

Nature of change

• Nature of change these are physical changes, so they are reversible.
• The number of molecules and the mass do not change; only the energy changes.
• All substances do this, but at different temperatures, so do not mention water unless asked.

Section 3 of 22Arrangement and motion of particles

Arrangement and motion of particles

Always in motion

• Always in motion all particles move at any temperature.

Expansion

• Expansion particles do not expand; the average separation changes.

Property	Solid	Liquid	Gas
Density	High	Medium	Low
Arrangement	Regular pattern	Random, no gaps	Random
Separation	Very close	Close, loosely packed	Widely spaced (about 10 times further apart)
Movement	Vibrate about fixed positions	Slide past each other	Fast and in all directions
Energy	Lowest	Higher	Highest

Section 4 of 22Forces between particles

Forces between particles

Solids

• Solids strong intermolecular forces hold particles in place, so they only vibrate.
• This gives a rigid shape and fixed volume.

Liquids

• Liquids particles have enough energy to move past each other but not to escape.
• The volume is fixed, but the liquid flows and changes shape.
• Liquids are hard to compress because particles are close and repulsion is large.

Gases

• Gases particles have overcome the forces holding them together and move randomly at high speed.
• Large gaps let the gas be compressed, expand and flow freely.

The word particles

• The word particles it can refer to atoms, molecules, ions or electrons.

Section 5 of 22Temperature and absolute zero

Higher temperature

• Higher temperature the average speed and kinetic energy of the particles are greater.

Lowest energy

• Lowest energy there is a temperature at which particles have the least kinetic energy, called absolute zero (−273 °C).

Section 6 of 22More key facts: States of Matter and Particle Arrangement

1Same particles

• Same particles a substance has the same particles in every state, only the arrangement and energy differ.

2Density

• Density solids and liquids are dense because their particles are close, while gases have a low density.

3Compression

• Compression gases can be squashed because of the large gaps between particles.

4Flow

• Flow liquids and gases flow because their particles can move past each other.

5Condensing and boiling

• Condensing and boiling they are opposite processes between gas and liquid.

6Energy change

• Energy change heating gives particles more energy and cooling takes it away.

Section 7 of 22More from the specification

1Gas Pressure and Molecular Collisions

• Gas pressure arises from continuous, random collisions of gas particles with the container walls; each collision exerts a microscopic force,
• and the total force per unit area on the walls is measured as pressure.
  • (in units of pascals, Pa)

2Boyle's Law at Constant Temperature

• For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume (p·V = constant);
  • compressing gas in a balloon or cylinder increases collision frequency with the walls, thereby increasing pressure.

3Absolute Temperature and Brownian Motion

• Absolute temperature measured in kelvin (K = °C + 273) is directly proportional to the average kinetic energy of particles;
  • Brownian motion provides experimental evidence for this continuous random kinetic motion as visible smoke or pollen particles collide with invisible fluid molecules.

4Inversely proportional

• One quantity falls as the other rises in the same ratio.

5Constant

• Not changing.

6Kelvin

• SI unit of temperature on the absolute scale.

7Collision

• Short interaction with large internal forces.

8Brownian motion

• Random jiggling of small visible particles hit by fluid molecules.

9Constant volume

• The container is rigid and sealed.

10p1V1

p1V1 p2V2

What the symbols mean: p₁, V₁ = pressure and volume before · p₂, V₂ = after; only for a fixed mass of gas at constant temperature

Section 8 of 22Motion of gas molecules

Random motion

• Random motion gas molecules move in constant random motion at high speed
• They travel in no specific path.
• They change direction suddenly when they collide with the container walls or with other molecules.

Temperature

• Temperature a hotter gas has faster molecules with more kinetic energy

Section 9 of 22How a gas exerts pressure

Cause of pressure

• Cause of pressure gas pressure is caused by molecules colliding with the walls of the container

Momentum change

• Momentum change when a molecule bounces off a wall its momentum changes direction, and this change of momentum over time produces a force on the wall
• The force acts at right angles to the wall (or any surface).

Pressure

Pressure force per unit area, p = F ÷ A

• p = pressure in pascals (Pa).
• F = force in newtons (N).
• A = area in metres squared (m²).

Steady pressure

• Steady pressure the very large number of collisions over the whole wall gives a steady pressure

Section 10 of 22Effect of temperature and compression on pressure

1Faster molecules

• Faster molecules at a higher temperature molecules collide with the walls more often and with greater force
• The pressure of the gas therefore increases.

2Compression at constant temperature

• Compression at constant temperature the average speed and kinetic energy stay the same
• Only the frequency of collisions per unit area changes, so the pressure increases.

Change	Effect on molecules	Effect on pressure
Temperature rises (constant volume)	Faster, so more frequent and harder collisions	Increases
Volume decreases (constant temperature)	Same speed but more frequent collisions per unit area	Increases
Volume increases (constant temperature)	Less frequent collisions	Decreases

Section 11 of 22Brownian motion

Observation

• Observation the Scottish scientist Robert Brown saw pollen grains in water moving randomly under a microscope

Definition

Definition the random movement of microscopic particles suspended in a liquid or gas

Smoke

• Smoke smoke particles in air show the same behaviour

Importance

• Importance it shows that matter is made of particles in constant motion, which is why the model is called kinetic

Section 12 of 22Explaining Brownian motion

Cause

• Cause small, light, fast-moving molecules of the liquid or gas collide with the larger visible particle
• Each collision gives the particle a small nudge that changes its speed and direction.
• At any moment more molecules strike one side than the other.
• The uneven bombardment gives a resultant force that pushes the particle in a random direction.

Inference

• Inference the molecules are too small to see, so their presence is inferred from the motion of the visible particles

Drawing the path

• Drawing the path use short straight lines of different lengths joined by sudden changes of direction, never curves

Naming

• Naming the motion is called Brownian motion, not diffusion

Section 13 of 22Factors affecting Brownian motion

Factors affecting Brownian motion

Particle size

• Particle size larger particles move less because the molecular bombardment is more evenly balanced

Temperature

• Temperature a hotter fluid makes the visible particles move more because its molecules are faster

Link to diffusion

• Link to diffusion diffusion also results from random molecular motion but is the spreading of a substance rather than the jiggling of a particle

Section 14 of 22More from the specification

More from the specification

Fixed particle geometry in solids

• Fixed particle geometry in solids strong intermolecular bonds fix vibrating particles into rigid crystalline arrays, defining the fixed shape of solids.

States of Matter Particle Model

• In a solid, particles are packed close together in fixed positions with strong intermolecular forces;
  • in gases, particles move freely with negligible forces.

Kelvin Temperature Scale and Absolute Zero

• Absolute zero (0 K or -273°C) is the temperature at which particles have minimum kinetic energy;
  • conversion requires T(K) = θ(°C) + 273.

Gas Pressure in a Balloon

• Gas particles collide elastically with the inner surface of an elastic balloon, exerting outward force per unit area that balances external pressure.

Boyle's Law for a Fixed Mass of Gas

• At constant temperature, the pressure and volume of a fixed mass of gas satisfy p₁V₁ = p₂V₂, provided pressure and volume are in consistent units.

Inversely proportional

• One quantity falls as the other rises in the same ratio.

Kelvin

• SI unit of temperature on the absolute scale.

Solid

• State with a fixed shape and a fixed volume.

Absolute temperature

• Temperature measured in kelvin.

p1V1

p1V1 p2V2

What the symbols mean: p₁, V₁ = pressure and volume before · p₂, V₂ = after; only for a fixed mass of gas at constant temperature

Section 15 of 22The kelvin scale

Absolute temperature

• Absolute temperature temperature in kelvin is called absolute temperature and begins at absolute zero
• 0 K is −273 °C.
• It is not possible to have a temperature below 0 K, so a kelvin temperature is never negative.

Size of a degree

• Size of a degree a rise of 1 K is the same change as a rise of 1 °C

Conversion

• T (K) = θ (°C) + 273 and θ (°C) = T (K) − 273
• 0 °C is 273 K.
• 20 °C is 293 K.

Section 16 of 22Pressure and volume at constant temperature

Compressing

• Compressing a smaller volume gives more frequent collisions with the walls, so the total force on each wall and the pressure increase

Expanding

• Expanding a larger volume gives fewer collisions, so the pressure decreases

Balloon in a bell jar

• Balloon in a bell jar a tied balloon holds a fixed number of air particles
• When air is pumped out of the jar, the pressure outside falls below the pressure inside.
• The spaces between the particles increase, so the balloon expands.

Section 17 of 22Pressure and temperature at constant volume

Heating

• Heating molecules move faster, so they collide with the walls more often and with greater force and the pressure rises

Cooling

• Cooling the molecules slow down and the pressure falls

Graph

• Graph pressure against kelvin temperature is a straight line through the origin, so pressure is directly proportional to absolute temperature

Pressure law

• Pressure law at constant volume p ÷ T is constant when T is in kelvin

Section 18 of 22Boyle's law

Statement

• Statement for a fixed mass of gas at constant temperature, pressure is inversely proportional to volume

Forms

• Forms p ∝ 1/V, or pV = constant, or p₁V₁ = p₂V₂
• p₁ and p₂ = initial and final pressure in Pa.
• V₁ and V₂ = initial and final volume in m³.

Units

• Units kPa and cm³ may be used instead of Pa and m³ if the initial and final units match

Graph shape

• Graph shape pressure against volume is a curve that approaches but never touches either axis, not a straight sloping line

Doubling the pressure

• Doubling the pressure at constant temperature this halves the volume

Section 19 of 22Conditions for the gas laws

Fixed mass

• Fixed mass the number of molecules of gas must not change

Constant temperature

• Boyle's law applies only when the temperature stays the same

Kelvin temperatures

• Kelvin temperatures use kelvin, not Celsius, in any proportionality involving temperature

Section 20 of 22More from the specification

More from the specification

Intermolecular bonding forces in solids

• Intermolecular bonding forces in solids tightly packed lattice arrangements and strong attractive forces maintain fixed geometric shapes in crystalline solids.

States of Matter and Particle Arrangement

• In a solid, particles vibrate about fixed positions;
  • in a liquid, particles are closely packed but slide past one another;
  • in gases, particles move freely at high speeds in random directions.

Kinetic Particle Theory of Matter

• The temperature of a substance is directly proportional to the average random kinetic energy of its constituent particles,
• where thermal energy input increases molecular vibration or translation.

Brownian Motion

• Brownian motion is the continuous, erratic, random motion of microscopic particles caused by uneven collisions with invisible fluid molecules.
  • (such as smoke in air or pollen in water)

Gas Pressure in Containers

• Gas pressure exerted on container walls arises from billions of rapid, random collisions of gas particles per second, transferring momentum and creating an average outward force per unit area.

Brownian motion

• Random jiggling of small visible particles hit by fluid molecules.

Random motion

• Movement in no specific path with sudden changes at collisions.

Kinetic energy

• K = ½mv², energy of motion.

Average speed

• Total distance divided by total time.

Liquid

• State with a fixed volume that takes the shape of its container.

Random

• Without a pattern in speed or direction.

Solid

• State with a fixed shape and a fixed volume.

p1V1

p1V1 p2V2

What the symbols mean: p₁, V₁ = pressure and volume before · p₂, V₂ = after; only for a fixed mass of gas at constant temperature

Section 21 of 22Key terms

Term	Meaning
Solid	A state with fixed shape and volume.
Liquid	A state with fixed volume that takes the shape of its container.
Gas	A state that fills its container and is highly compressible.
Intermolecular force	An attractive force between particles.
Absolute zero	The lowest possible temperature, −273 °C, where particles have least kinetic energy.
Condensing	A gas turning into a liquid.

Term	Meaning
Pressure	Force per unit area, p = F ÷ A, measured in pascals (Pa).
Brownian motion	Random movement of microscopic particles suspended in a liquid or gas.
Random motion	Motion in no specific path with sudden changes of direction at collisions.
Kinetic model	The idea that all matter is made of tiny particles in constant motion.
Momentum change	Change in the momentum of a molecule when it rebounds from a wall, producing a force.

Term	Meaning
Absolute zero	The lowest possible temperature, 0 K or −273 °C.
Kelvin	The SI unit of absolute temperature; a 1 K change equals a 1 °C change.
Boyle's law	For a fixed mass of gas at constant temperature, pV is constant.
Inversely proportional	One quantity halves when the other doubles.
Directly proportional	Two quantities increase together in a constant ratio, giving a straight line through the origin.

Section 22 of 22Exam technique

1

Saying particles expand when a substance is heated — the separation increases, not the size of the particles.

2

Saying particles in a solid do not move — they vibrate.

3

Writing "water" in answers about states when no substance is named.

4

Saying mass changes during melting or boiling.

1

Saying the gas molecules themselves are seen in Brownian motion — only the larger particles are seen.

2

Drawing a curved path for a Brownian particle — use short straight lines with sudden changes of direction.

3

Saying temperature changes the number of molecules in a sealed container — it changes their speed.

4

Mentioning collisions without saying with the walls — the collisions with the container walls cause the pressure.

1

Using Celsius temperatures in a proportionality relationship — use kelvin.

2

Drawing the Boyle's law graph as a straight sloping line — it is a curve approaching both axes.

3

Forgetting that the mass and temperature of the gas must stay fixed for Boyle's law.

4

Mixing kPa with m³ in the same equation — keep consistent units on both sides.